Reverse Cholesterol Transport (RCT) is the movement of excess cholesterol from peripheral tissues to the liver for disposal as biliary cholesterol and bile acids. In recent years, much research has focused on finding ways to enhance this process so as to both prevent and treat cardiovascular disease. High density lipoproteins (HDL) are the primary vehicles for RCT and transport the cholesterol primarily as cholesteryl esters. The overall goal of this proposal is to better understand the mechanisms by which hepatocytes selectively remove and hydrolyze cholesteryl esters from HDL and deliver the free cholesterol to bile or first metabolize it to bile acids. Particular focus is given to study of the enzyme(s) responsible for hydrolyzing these cholesteryl esters and the metabolic changes that ensue if this hydrolysis does not occur. Carboxyl ester lipase (CEL), also called cholesterol esterase and bile salt-stimulated lipase, is made and secreted by the liver. Preliminary results from cultured cells and CEL-null mice show that this enzyme plays a role in both the selective uptake and the subsequent hydrolysis of HDL cholesteryl esters. Three specific aims are proposed. 1) Characterize the physiological consequences of the absence of CEL and investigate the mechanism by which CEL affects hepatic cholesterol metabolism. Cholesterol and bile acid synthesis as well as lipoprotein metabolism will be studied in CEL-null mice. 2) Test the hypothesis that increasing hepatic CEL expression will increase selective uptake and delivery of HDL cholesterol to the bile using transgenic mice with elevated liver expression of CEL. 3) Test the hypothesis that removing CEL from mice that express CETP (cholesteryl ester:triglyceride transfer protein) will increase lipid exchange between HDL and triglyceride-rich lipoproteins and result in more rapid removal of the latter from the circulation. The CETP gene will be transferred to CEL-null mice by cross-breeding with mice that express a simian CETP transgene. Relevance to Public Health: The results of the proposed experiments will provide valuable information about HDL function and liver cholesterol metabolism that may help identify new pharmacologic targets for increasing cholesterol breakdown and excretion, thereby reducing the risk for and progression of cardiovascular disease.